1. Field of the Invention
Heretofore, suspension fertilizers produced batchwise by the direct ammoniation of solid intermediates derived from wet-process orthophosphoric acids or combinations of such solid intermediates and wet-process or other impure phosphoric acids were found to be relatively unstable because the acid congeneric metallic cation impurities caused the formation of strong gels during storage which gels could not be broken unless the suspension was diluted to a lower grade, and in some cases even dilution did not result in a product having enough fluidity to be conveniently handled. In order to produce suspensions that were fluid and easily handled during the complete storage-distribution cycle, relatively low-grade products had to be produced in order to avoid such high viscosity and low-pourability problems. These low-grade products presented a very distinct disadvantage because of transportation and handling as well as application costs together with reduced effective producer storage capacity, which reduced effective storage capacity was or is a substantial concern during the critical period comprising the spring rush season.
2. Description of the Prior Art
Prior art suspension fertilizer production technology is well known and fully described in the literature. See, just for example, the following references, most of which are assigned to the assignee of the present invention, and are discussed in greater detail, infra:
______________________________________ U.S. Pat. No. Author Issue Date ______________________________________ 3,019,099 Walters 01/30/62 3,813,233 Kendrick 05/28/74 3,711,268 Frazier 01/16/73 3,861,897 Frazier 01/21/75 4,066,432 Jones 01/03/78 4,375,980 Jones, et al. 03/08/83 4,383,847* Barber 05/17/83 4,511,388 Jones, et al. 04/16/85 ______________________________________ *Not assigned to the assignee of the present invention.
Examples of other literature published by investigators and researchers of the assignee of the present invention and dealing with complexing impurities salts in suspension fertilizers are:
Dillard, et al. "Precipitated Impurities in Monoammonium Phosphate and Their Effect on Chemical and Physical Properties of Suspension Fertilizers," TVA Bulletin Y-183, NFDC, Muscle Shoals, Ala.
Dillard, et al. "Precipitation of Impurities in 9-32-0 Grade Fluid Fertilizers," TVA Bulletin Y-194, NFDC, Muscle Shoals, Ala.
Frazier, et al. "Solubilities in the System Magnesium Oxide-Ammonia-Orthophosphoric Acid-Pyrophosphoric Acid-Water at 25.degree. C.," TVA Bulletin X-507, NFDC, Muscle Shoals, Ala.
Jones, et al. "High-Grade Suspensions Produced from Wet-Process Orthophosphoric Acid," NFDC, Muscle Shoals, Ala.
Phosphate base suspension fertilizers are commonly produced by the direct ammoniation of phosphoric acids or slurries of solid intermediates derived from monoammonium phosphate (MAP). During the last several years, most ammonium phosphate fluid fertilizers having desirable storage properties contained substantial proportions of the P.sub.2 O.sub.5 present as polyphosphates. Polyphosphates have long been known to increase the solubility of the phosphate salts and aid in the sequestering of the congeneric metallic impurities that otherwise form gel-like compounds which compounds tend to destroy the fluidity of the resulting concentrated fluid fertilizers, U.S. Pat. No. 3,015,552, Striplin, et al., Jan. 2, 1962, assigned to the assignee of the present invention. In most cases, such fluid fertilizers were of the solution-type and consequently contained no undissolved solids. The cost for fluid fertilizers containing polyphosphates has been relatively high, and as a result, such fluid fertilizers have experienced difficulty in competing with solid fertilizers. Generally, wet-process orthophosphoric acid (50-56% P.sub.2 O.sub.5) costs less per unit of P.sub.2 O.sub.5 than merchant-grade superphosphoric acids (68-70% P.sub.2 O.sub.5), containing polyphosphates. However, even with such lower costs the overall role of orthophosphoric acid as the sole phosphate source in the production of suspensions has remained somewhat limited over the years. The principle reason for such limited utilization of ortho acid has been the economically restrictive relatively low grades which can be produced from them because of the presence of congeneric impurities such as iron, aluminum, magnesium, and calcium, which impurities post precipitate as amorphous compounds with gel-like characteristics after such acids have been ammoniated. These gel-like compounds completely destroy the fluidity of high-analysis suspension fertilizers and make it necessary to dilute same with water to lower the grades thereof to avoid gelation or severe thickening during production, transportation, storage, and/or distribution. In the past when high-grade suspensions were produced from such low-cost, merchant-grade wet-process phosphoric acids, severe problems were encountered with transfer pumps, pipelines, tanks, and valves when such suspensions, being in a semisolid condition, were being produced or otherwise handled. It has oftentimes been the experience of many fertilizer dealers that when such suspension products become semisolid, it is extremely difficult and expensive to restore their fluidity to the point where they can be used either for direct application or in the subsequent production of mixed fertilizer grades by adding thereto a source of nitrogen and/or potash.
A typical method for producing suspension fertilizers at the dealer level is by the batch ammoniation of MAP. Such batch processing is economical and relatively straightforward and can be retrofitted in production systems and operations which most dealers have and presently utilize. MAP is a high-grade solid intermediate (11-52-0 grade) produced from merchant-grade orthophosphoric acid (normally 54% P.sub.2 O.sub.5) by using any of numerous granulation processes known in this art. It therefore contains all of the impurities (congeneric) that were originally present in the starting wet-process acid. MAP is an attractive raw material because it is widely available and easy to store. Most suspensions made from MAP are produced in the early spring or late winter and therefore must be stored for periods of time ranging upwards to 3 months. These suspensions are then transferred to cold-mix equipment to produce the desired mixed fertilizer grades. Severe problems often occur during this long storage period, with the product becoming thixotropic and very difficult to handle. This is due to the impurities forming gelatinous compounds which destroy fluidity. When such gel formation occurs, the only viable approach available to the dealer has been to dilute the suspension with water to thereby lower the grade. This encroaches upon otherwise valuable time during the rushed peak season. The same problems occur and the same approach is necessary when wet-process orthophosphoric acid is used as the phosphate source or if combinations of MAP and wet-process orthophosphoric acid are used. Until the instant invention and its related technology was developed, the only way known for increasing the fluidity of such orthophosphate suspensions was by means of dilution of same with water, which resulted in unwanted reduction in product grades.
In 1962, Walters ('099, supra) produced 8-24-0 grade ammonium orthophosphate suspensions by both batch- and continuous-type operations by the ammoniation of wet-process orthophosphoric acids. In this work, Walters found that the impurities in the fairly clean wet acids he was dealing with caused a thixotropic characteristic which prevented the production of higher grade suspensions because of excessively high viscosities. Walters used these gel-like compounds to prevent settling of the ammonium phosphate crystals in his relatively low-grade suspension fertilizers.
In 1974, Kendrick ('233, supra) identified techniques and methods in which impure wet-process orthophosphoric acid can be ammoniated without producing gel-like impurity compounds which compounds caused complete destruction of the suspension fluidity. Kendrick's teachings involved the continuous ammoniation of the wet acid in two separate stages. By means of such two-stage processing he was able to produce much higher grades (such as 11-39-0 versus 10-30-0) having good storage properties. He also made similar products by a batch simultaneous procedure which procedure, he indicated, did not work quite as well as did his continuous procedure.
In 1977, Jones ('432, supra) improved on Kendrick's teachings by developing a three-stage process for the production of satisfactory high-grade suspensions through the ammoniation of wet-process orthophosphoric acids under conditions in which gel-like impurity related compounds did not form. In this work, the product (13-38-0 grade) crystals did not settle or pack during transit due to the vibrational energy that occurs especially during shipment by rail as was the case in products produced by Kendrick's teachings. In 1984, Jones, et al., ('388, supra) developed a batch procedure based on the same phenomena in which he ammoniated wet-process orthophosphoric acids directly by using a heel of hot product from a previous batch to provide the nuclei and environment in the reactor for making metallic impurity crystalline compounds instead of the metallic impurity gel-like compounds which gels cause destruction of the suspension fluidity. A significant negative trade off of this procedure was the reduction of the net volume of each batch by at least one-third because of the necessity of maintaining such heel. Also, the procedures of Jones, et al., did not completely stabilize the suspension during long-term storage.
Until the development of the present invention, all efforts to produce high-analysis ammonium orthophosphate suspensions having satisfactory storage properties by stabilizing the congeneric impurities stemming from wet-process phosphoric acid with the addition of fluorine-containing compounds have either not been successful or have not been adopted by the fluid fertilizer suspension industry, it being appreciated that the use per se of fluorine in research is not new or novel regarding fertilizers of either the solid or fluid form.
As early as 1973, Frazier ('268, supra) found that a small excess of the fluoride ion over that required to react with all the aluminum and magnesium in ammonium and potassium polyphosphate liquid fertilizer solutions would prevent the post precipitation of the wet-acid congeneric impurities during product storage. Inversely, a smaller amount of fluorine added allowed precipitation of the metallic cations so that they could be filtered out to obtain a stabilized solution.
The instant, new, and novel invention teaches a critical sequence of fluorine addition to effectively decrease the resulting water-insoluble solid contents of product suspensions and does not effect an increase in post precipitation as a mechanism to remove the impurities in solution. In addition, the present invention deals with the production of suspension fertilizers and not true solutions as does Frazier's teachings.
Later in 1975, Frazier ('897, supra) again developed a procedure in which a small excess of fluoride was added to prevent precipitation of aluminum and magnesium by using the fluoride ions to sequester the metal cations.
The instant, new, and novel procedure differs once again from this later work of Frazier in that it is concerned with producing suspensions and not true solutions. Also, referring to Example IV, infra, fluoride levels used in Frazier's work with fluorosilicic acid as the fluoride source for solutions are too low and would not be applicable to use in suspensions because at the levels shown necessary for practice of this invention, Frazier's teachings indicate that the pourability is significantly decreased by the formation of a silica gel during storage of his solutions.
In 1983, Jones, et al. ('980, supra), developed a process in which small proportions of fluorosilicic acid (0.1% to 0.5 by weight percent) were added, in the production of suspensions made from wet-process orthophosphoric acid or other impure acids, to cause modifications of the resulting ammonium phosphate crystals. This phenomena prevented excessive settling during transit caused by vibrational energy. Jones, et al., teach that the fluorosilicic acid which is introduced into the mixing vessel is proportioned with his resulting cooled ammoniated material introduced therein to effect a fluorosilicic acid concentration in his resulting stable suspension of about 0.1 to 0.5 weight percent. The present invention differs from Jones, et al's teaching in the following two respects. First, in the instant, new, and novel process, operation is at higher fluoride levels than Jones, et al., teach can, be used. For instance, Jones, et al., teach that if more than about 0.5 weight percent H.sub.2 SiF.sub.6 is used in his third stage (after all ammoniation is complete) this would cause formation of SiO.sub.2.H.sub.2 O gels which gels would completely destroy the fluidity of his suspension fertilizer. Secondly, work by Jones, et al., revolves around adding the fluorisilicic acid after the ammoniation is completed and the product is cooled (100.degree. to 130.degree. F.) whereas in the instant new procedure, the fluorosilicic acid is added prior to the ammoniation step(s) with the temperature ranging from 165.degree. to 210.degree. F., see Example IV, infra.
In a 1983 publication entitled "A Waste Recovery Story, Barber, et al., discuss the use of wastewater which contains fluorine to use as a substitute for make-up water in the overall formulation. As with Jones, et al. ('980, supra), the fluorine was added after ammoniation and after the product was cooled. Again only 0.4 weight percent fluorosilicic acid was added. As discussed in Example IV, infra, these procedures would cause destruction of suspension fluidity. One interpretation of the teachings of this publication is that insufficient fluorine is available in the wastewater to provide the levels that are needed to improve the physical properties of the suspension fertilizers so extra wastewater is added to the cooled product to improve the viscosity and pourability of the suspension. This appears quite contrary to the findings leading to development of the present invention which indicate that when fluorosilicic acid is added as the fluoride source to the cooled product, strong silica gels are formed which destroy suspension fluidity. Accordingly, the investigators who developed the present invention are not throughly convinced that such process, as taught by Barber, et al., just supra, can be used for the continuous production of suspension fertilizers. The procedure of the instant invention differs from that of Barber, et al., in two ways. First in its principally intended environment, it uses batch-type processing whereas Barber, et al., describe a continuous-type process. Secondly, the fluorosilicic acid is added prior to the instant ammoniation step without any attendant fluorine evolution and such instant ammoniation step is subsequently initiated within a relatively short time thereafter, about 1 to 5 minutes, and the subsequent ammoniation step is also quickly completed, i.e., usually in about 15 minutes. In Barber, et al.'s, publication, supra, they teach adding fluorine to their first-stage reactor during ammoniation at boiling temperatures of about 230.degree. F. with no mention of attendant fluorine evolution. Based on the data leading to Example V, infra, substantial evolution of fluorine would occur using Barber, et al.'s, procedure, and would be expected since the first-stage reactor is operated at a pH of between 4.5 and 5.0 and at a temperature of about 230.degree. F. (boiling) with a retention time of greater than 30 minutes. Data indicate that about 30 to 36 percent of the fluorine added in this first-stage of Barber, et al., would be evolved. On the other hand, the instant new batch procedure shows no fluorine loss during the quickly effected ammoniation step.